Replace the placeholder "UNLICENSED" with the MIT License so the now-public
repo is genuinely free and open source. Adds a LICENSE file and documents the
app-vs-bundled-binary licensing split in the README (yt-dlp/FFmpeg/aria2 are
invoked as separate executables, so they remain under their own licenses).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
